Power regulators are typically used to regulate power flow in power transmission systems that transfer power among utilities or between a utility and its consumers. The transmission system transfers alternating current ("AC") power through a polyphase transmission line, such as a three phase line with each phase having a voltage displaced in phase with respect to the other phases by 120.degree.. A conventional power regulator is placed in series with the line between a source of power, such as a generator, and a load, such as the consumer(s) or another utility system, to control power flow.
By controlling power flow in the transmission line, the power regulator may improve the stability of the transmission system. Stability is the property of a power system that ensures that the system will operate in equilibrium under normal conditions, and quickly return to equilibrium after a disturbance. For example, when two utilities are linked together, the transmission system interconnecting the utilities becomes unstable if a generator in one of the utilities falls out of synchronism. Severe disturbances along the transmission line may alter the power flow in the line, causing the generators to lose synchronism, and the power system to become unstable.
Conventional power regulators address two forms of stability: steady state and transient stability. Steady state stability is concerned with slow or gradual fluctuations in power flow. Transient stability addresses severe disturbances in the power flow, such as those caused by sudden changes in the load, switching circuits, or fault conditions. Fault conditions may occur during high winds and storms when power lines break, become entangled, or become shorted to ground.
Power regulators enhance the capacity of a transmission system to transfer power without exceeding stability limits. The stability limit of a transmission system is the maximum amount of power flow that the system may transfer without experiencing a loss of stability. A system has a steady state limit for gradual fluctuations in the power flow, and a transient stability limit for abrupt changes in the power flow. An abrupt disturbance, such as a severe fault on the line, may create a low frequency power swing along the line. Low frequency power fluctuations may cause the system to exceed its transient stability limit. The practical result is that several generation stations may trip off line (shut down) in response to these fluctuations, and consequently cause cascading blackouts over entire regions. By regulating power flow among the systems, the occurrence of such regional blackouts may be reduced or avoided.
When interconnecting two or more utilities with a large phase angle difference between their respective transmission systems or networks, there are generally two viable alternatives for enhancing the stability of the network: back-to-back high voltage direct current ("HVDC") systems, and phase angle regulators. Back-to-back HVDC systems convert incoming AC power to direct current ("DC") power and then convert the DC power back to AC power. During this power conversion process, the back-to-back HVDC system may modulate the power transfer to damp any low frequency oscillations resulting from abrupt disturbances on the transmission line. While back-to-back HVDC systems provide acceptable steady state and dynamic control, they are very costly due to the expense of the power electronic converter and transformers included in the system.
The second alternative for enhancing network stability uses a phase angle regulator. Phase angle regulators may perform both steady state and dynamic control by modulating the phase angle in the transmission system to control power flow. Coupled to a controller which senses changes in power flow, the phase angle regulator compensates for these changes by adjusting the phase of the voltage between the source and load. In a balanced three-phase transmission line, the power delivered by a three phase source equals three times the average power in each phase. The average power in each phase is the product of the magnitudes of the line voltage and current and the power factor. The power factor is the cosine of the angle between the line voltage and current. A phase angle regulator shifts the phase angle of the voltage relative to the current to control power flow in the transmission line. The injection of the quadrature voltage results in a new line voltage shifted in phase from the original line voltage. Depending upon the magnitude and phase of the injected voltage, the phase angle regulator may increase or "boost" the voltage amplitude, decrease or "buck" the voltage amplitude, or simply provide a phase shift without significantly affecting the voltage amplitude.
In general, phase angle regulators inject a voltage by using transformers having two or more windings magnetically coupled together. Current passing through a primary winding of the transformer induces a current in, and a voltage across, a secondary winding. The induced voltage and current are related to the primary voltage and current by the turns ratio between the primary and secondary windings. Using this basic principal, the phase angle regulator typically has a first transformer which obtains a voltage from one phase of a transmission line, and a second transformer which adds the voltage to another phase of the line. One phase of the primary winding of the first transformer is typically coupled in parallel with one phase of a polyphase transmission line, and the secondary winding of the second transformer is typically coupled in series with another phase of the transmission line. A secondary winding of the first transformer is coupled to the primary winding of the second transformer to inject a voltage in series with a phase of the transmission line. In such systems, the magnitude of voltage injected may be adjusted by altering the turns ratio of the windings. To alter the number of turns in a winding, conventional phase angle regulators have a switching mechanism, called a "load tap changer," to switch among contacts, called "taps," along the winding.
Conventional phase angle regulators provide steady state control. Typically, such conventional systems use mechanical load tap changers that are significantly less costly than the power electronics used in back-to-back HVDC systems. One significant drawback of these conventional systems is the lack of adequate dynamic phase angle regulation because mechanical tap changers cannot switch fast enough to respond to abrupt disturbances in the transmission system. When switched quickly, the mechanical tap changers generate high losses which are manifested in arcing. As a consequence, conventional phase angle regulators are not capable of providing suitable dynamic phase angle regulation.
To address this drawback, conventional phase angle regulators use semiconductor switching devices, such as thyristors, capable of very fast switching with very low losses. Such semiconductor switch devices are known as "static" devices for their lack of physical motion as compared to mechanical tap changers. One should not confuse this terminology with the term "dynamic," which refers to the very rapid phase shifting performed by phase angle regulators. With thyristor control, phase angle regulators can provide both steady state and dynamic phase angle regulation necessary to regulate power flow and maintain the stability of interconnected power systems.
However, as with back-to-back HVDC systems, the hardware in these earlier thyristor controlled phase angle regulators is very expensive. Both the physical size and cost of the components are directly related to the through-current rating of the transmission line and its rated voltage. If the phase angle regulator is to provide continuous steady state control, then the components must be rated for continuous full load current operation. If such a steady state phase angle regulator is to provide phase angle regulation during a fault, then the components must be rated to withstand much higher voltage stresses and fault current levels. Finally, the components in such phase angle regulators must have ratings roughly proportional to the maximum phase angle because higher phase angles require the injection of higher voltages, which places more stress on switching components. Because steady state regulators must operate continuously and must provide large phase shifts, the regulator components require higher ratings, and consequently, are more expensive.
One earlier thyristor controlled phase angle regulator was proposed in 1982 by R. Baker, then of Washington State University, and G. Guth, then of the Brown Boveri Corporation, and is referred to herein as "the BBC regulator." While the BBC regulator provides the necessary stability enhancement, it is not cost effective for many applications because it includes costly power electronics and two separate transformers. The BBC regulator has a first transformer with primary windings coupled between each phase of a three phase system, and a secondary winding for each primary winding. Each phase of the secondary winding has coil segments with a 1:3:9 turns ratio relative to each primary winding.
The BBC regulator has a bank of thyristor switches which selectively couple the secondary windings of the first transformer to primary windings of the second transformer. The second transformer includes secondary windings coupled in series with respective phases of the transmission line. By switching a coil segment of the secondary of the first transformer, the phase angle regulator may inject a range of discrete voltages at the second transformer in series with the transmission line. This range of discrete voltages provides smooth transitions over the entire range of operation, and thus, is known as "vernier" phase angle regulation.
While the BBC regulator is superior to conventional phase shifters because it provides dynamic phase angle regulation, the hardware included in the BBC system makes it unduly expensive. Both the large number of thyristors included in the BBC switch bank, and the two separate transformers, increase the cost of the BBC design. Each of the BBC thyristors must be rated for continuous operation if the BBC regulator is to provide both steady state and dynamic control. Moreover, the two separate transformers in the BBC design each require separate cores and tanks. For moderate to larger phase angle differences, on the order of 25.degree. or greater, the BBC system can be as costly or more expensive than the back-to-back HVDC systems. For small phase angle differences, in the range of 25.degree. or less, the BBC static phase shifter has a cost advantage over the back-to-back HVDC system.